The earliest retroreflective sheeting had an exposed-lens construction, but its retro-reflective light was blanked out when the lenticular surface of the exposed microlenses was covered with water. This problem was answered by enclosed-lens or embedded-lens retroreflective sheeting in which, as first taught in U.S. Pat. No. 2,407,680 (Palmquist et al.), a monolayer of the microlenses were embedded within the sheeting which had a flat, transparent cover film. This allowed incident light rays to be focused onto the specularly reflective layer irrespective of whether the front of the sheeting was wet or dry.
Another type of enclosed-lens or embedded-lens retroreflective sheeting is taught in U.S. Pat. No. 4,511,210 (Tung et al.). That sheeting may be made from a retroreflecting base material as shown in FIG. 6 including sequentially a layer 25 of pressure-sensitive adhesive, an aluminum specularly reflective layer 24, a transparent spacing layer 23, and a monolayer of glass microspheres 21 partially protruding from the front surface of the sheet material. "Thereafter an oriented polymethylmethacrylate film 26 coated with a layer of pressure-sensitive acrylate adhesive 27 was pressed against the microsphere-exposed surface, embedding the microspheres into the adhesive and forming a transparent front layer on the sheeting in the manner shown in FIG. 2" (Col. 6, lines 40-45). Before applying the polymethylmethacrylate film, the retroreflecting base material was optically incomplete, and its elements were selected so that the application of the flat polymethylmethacrylate cover film would complete the optics.
In addition to the optical role of the cover film, it should protect the underlying elements of the retroreflective sheeting against adverse effects of ultraviolet radiation and moisture. It also should have a smooth surface that is highly transmissive of light, should resist dirt accumulation, and for some uses should be inkable. The cover film also can lend flexibility and extensibility to the retroreflective film or it can make the film stiffer and less flexible.
The flat, transparent cover film of the Palmquist patent can be provided either by a coating such as the acrylic polyester of its Example 1 or by a preformed plastic film. The flat, transparent cover film used in Example 1 of the Tung patent is a preformed polymethylmethacrylate film 26 which has excellent clarity and weatherability and doesn't pick up dirt. However, it is quite brittle and often cracks when the retroreflective sheeting is flexed or stretched such as when the sheeting is being conformed to the head of a rivet or embossed after being adhered to a license plate blank. To afford retroreflective sheeting of better flexibility and stretchability, other materials have been used as transparent cover films such as alkyd resins or plasticized polyvinyl chloride. Cover films of these materials have less resistance to weathering and are more likely to become contaminated by dirt.
U.S. Pat. No. 3,190,178 (McKenzie) solved in a different way the problem of retaining retroreflectivity while wet. Its solution involved modifying retroreflective sheeting of the exposed-lens type wherein the lenses are partially embedded in a binder layer. As described in that patent, the exposed lenses are protected by a flat, transparent cover film to which the binder layer is sealed along a network of interconnecting lines to form a plurality of hermetically sealed cells within which the microspheres are encapsulated and have an air interface. Such wet-reflecting sheeting is called "encapsulated-lens retroreflective sheeting". Its transparent cover film must be a preformed plastic film which should serve the same needs and present the same problems as those mentioned above for enclosed-lens or embedded-lens retroreflective sheeting, except that the base material underlying the cover film of encapsulated-lens retroreflective sheeting is optically complete.
Another type of wet-reflecting retroreflective sheeting is prismatic rather than lenticular. The prismatic sheeting typically is molded plastic, one flat surface of which is exposed to the elements. The plastic is selected primarily for transparency and dimensional stability, but also to provide good resistance to weathering and dirt accumulation. By applying a transparent cover film over the molded plastic, selection of the plastic is no longer constrained by resistance to dirt accumulation, and the plastic need not have such good weathering resistance. A suitable prismatic retroreflective sheeting is disclosed in U.S. Pat. No. 4,025,159 (McGrath).